Toyota's chief quality officer and a top engineer will testify before a Senate committee on Tuesday in the third congressional hearing on the automaker's problems with unintended acceleration in its cars.

Members of the Senate commerce committee are expected to bear down on the question of whether the company has fully diagnosed what has caused some of its cars to speed out of control, and whether Toyota has dealt fairly with consumer complaints.

But as the third hearing approached, some in the industry were questioning whether Congress would get very far in exploring this basic dispute.

"No matter how hard the questioning at [the] Senate hearing, what won't be forthcoming is the answer we all need to hear: What is behind this malady of unintended acceleration affecting all manufacturers and of such concern to consumers?" Jeremy Anwyl, chief executive of Edmunds.com, an automotive Web site, wrote to Transportation Secretary Ray LaHood.

The first two hearings on the subject, held last week by the House Oversight and Government Reform Committee and the House Energy and Commerce Committee, alternated between members of Congress who closely questioned Toyota's top executives and those who suggested that the world's largest automaker has faced unfair criticism, ginned up by trial lawyers and the United Auto Workers union.

The National Highway Traffic Safety Administration has linked unintended acceleration in Toyotas to at least 34 deaths.

The automaker says it has resolved those problems by changing floor mats and replacing sticky accelerator pedals. But some safety advocates have suggested that the problem lies in the car electronics that now operate accelerators.

"If driver error is the issue, let's say so once and for all," Anwyl wrote. "If design issues are involved, let's identify and fix them. If there is a workaround, let's find it."

LaHood and NHTSA administrator David Strickland are also scheduled to appear before the commerce committee.

Critics have said federal regulators too willingly accepted Toyota's insistence during years of investigations that there was no problem.